Method of stretching hollow metallic bodies and apparatus therefor



April 6, 1965 E. wElss 3,176,493

l METHOD oF sTRETcHING HoLLow METALLIC BODIES AND APPARATUS THEREFOR Filed Sept. 25. 1961 3 Sheets-Sheet l A rroRA/E vs April 6, 1965 E. wE-:lss 3,176,493

METHOD 0F STHETCHING HOLLOW METALLIC BODIES AND APPARATUS THEREFOR Filed sept. 25. 1961 3 Sheets-Sheet 2 EUGENE WEISS l/vnwro(7 By MM, www

ATTORNEYS April 6, 1965 E. wElss 3,176,493

METHOD OF S TCHING HOLLOW METALLIC BODIES APPARATUS THEREFOR Filed sept. 25, 1961 Y s sh eets-Sheet 3 lnvenor E @ne Weiss by WM) tig( WM Arfamys United States Patent O 3,176,493 METHOD F STRETCHING HOLLOW METALLlC BODES AND APPARATUS TIEREFGR Eugene Weiss, y:i060 Park Ave., New York, NSY. Filed Sept. 25, 1961, Ser. No. 140,433 4 Claims. (Cl. 72-209) The present invention relates to a method of stretching hollow metallic bodies, particularly seamless tubes, over a mandrel in staggered passes composed of loose or driven rolls and apparatus for carrying out the said method.

One known method which was frequently used in various countries before and dur-ing the last World War, required a number of passes equal to or larger than the ratio of elongation, i.e. the ratio of the length of the stretched Work to its original length. The proiles of the passes which were used in this known method approximated circles, but in the individual passes only small stretchings were effected in order to avoid the development of ns at the gaps between the rolls. The number of passes which -are used today in push benches has become somewhat smaller, but it is :still close to the ratio of elongation and has remained practically unchanged in tube rolling mills, still being greater than the ratio of elongation. The demand for ever larger production, for ever longer tubes, yand ever lower manufacturing costs requires new methods in order to satisfy these desiderata.

There are also known methods in which a plurality of passes are used, the rolls of which have convex or straight profiles, in which larger stretchings can be the present invention provides a way to force a considerable part of the spread as it develops from the transverse direction to the longitudinal direction of the work, for reducing the remaining part of the spread in the transverse direction to an extent, which even for `the largest stretchings is too small to produce ribs reaching the entering circumference of the work, hence excluding completely the danger of iin formation. This is achieved, according to the invention, in the passes preceding the ones in which the finished wall thickness of the work is reached on the work circumference or on parts of the circumference, on the one hand by causing the predominant, and with respect to the longitudinal axis of the work the circular or noncircular, middle sections of the roll profiles to penetrate into the work to a depth such that the geometric mean ratio per pass,

also/sn is at least 1.26, which geometric mean ratio is computed from the ratio .s0/sn, in which so is the smallest wall thickness of the work entering the first of the n .passes and sn is the smallest wall thickness of the work after being rolled in the nth pass of these passes. This result is furthermore achieved by causing the end sections of the roll proiiles joined at points of inflection to the middle sections to penetrate into the work to a depth which, in

the case where there is a circular middle section with respect to the longitudinal axis of the work, which is equal to the depth to which the respective middle section penetrates, and where there is a middle section achieved as compared with passes with rolls having concave profiles throughout. Such a method is useful only as long as the ratio of the wall thickness of the work to the diameter of the mandrel remains large enough. However, as the Wall thickness decreases in each pass of the series, and thus the said ratio also constantly decreases, the stretching capacity of the series of passes as a whole becomes unsatisfactory.

It is well known in the art that the increase of the stretchings in passes composed of the usual number of rolls per pass, is limited because the spread grows too Patented Apr. 6, 1965 large, causing the danger of iin formation at the gaps between the rolls. This danger Ialso exists in known methods, the passes of which are composed of the usual number Yof rolls having profiles consisting of predominant concave middle sections, circular or non-circular with respect to the longitudinal axis of the work, and convex roundings as end sections, which roundings are joined at points of inflection to the cincular or non-circular middle zones, and in which passes longitudinal ribs are produced between the adjacent middle zones of the rolls. It is a known fact .that the stretchings in these passes are also limited by the danger of n formation.

In contradistinction to the known methods used to date in practice, in which method excessive stretchings result in work material penetrating into the gaps between the rolls, which is non-circular with respect to the axis of the work, to a depth somewhat less than the maximum depth to which the respective middle section penetrates, the difference in depth being at the most equal to one fourth of the diiference between the smallest wall thickness of the work prior to rolling in the respective pass and the smallest Wall thickness after it is rolled in the pass. Rolling pressure is exerted by the neighboring end sections of adjacent rolls in oblique directions from both side-s Iagainst the parts of the Work lying between adjacent middle sections of neighboring rolls as well as against the ribs developing on these parts caused by spread, for diverting considerable parts of the spread, as it develops, from the transverse direction to the longitudinal direction of the work and to reduce thereby the remaining part of the spread, thus precluding -any possibility of iin formation.

The proiles of the end sections of the rolls are preferably convex, but may also be partly convex changing to straight or may be straight only, and may be attached to the middle sections with or without small roundings at vthe points of inflection. The end sections of the roll pro- `files should be as narrow as possible, but not too narrow, in order that the small ribs developing on the work between neighboring end sections of adjacent rolls do not become too steep for being rolled down in the succeeding staggered pass. j Y

It is pointed out that the ratio of the smallest wall thickness prior to the rolling in each of the passes versus the smallest wall thickness of Ithe work after rolling in each of the respective passes following each other, in principle could be equal to the computed geometric mean ratio t'J'S'D/Sn this ratio being at least 1.26. However, this possibility can hardly be achieved and could by no means be maintained in practice. The passes in a rough hot rolling mill are never accurate, and funthermore, the wear of the rolls in the different passes as well as of the mandrel is irregular.

The invention will now be described more particularly, reference being made to the accompanying drawings, in which:

FIG. 1 is an axial end view of a three roll pass showing the mandrel in section;

FIG. 2 is a similar view of a six roll pass showing the workpiece in the pass;

FIG. 3 is an axial end view of aV three roll pass with rolls having a modified prole; and

FIG. 4 is a view similar to FIG. 3 of a pass with rolls having a dii-ferent modi-hed proiile.

As seen in FIG. l, the roll pass comprises a mandrel b i of the pass or has a concave shape slightly differing towards the ends from the said circle, and of two end sections, each having a profile f-g, which sections are joined at points of inilection f to the ends of the middle sections f-f. The end points f of the middle sections are points of inflection since` the tangent to the middle sec- Vtions drawn at these points showsthat the said middle sections and the end sections joined thereto lie on the opposite side of the tangent. Thus, the roll profile changes direction at the said point. The end sections f-g penetrate into the work to a depth equal to the depth the middle sections penetrate if they lie on the inscribed circle of the pass, or they penetrate into the work to la depth nearly equal to the depth the middle sections penetrate if they have a concave shape slightly differing towards their ends from the inscribed circle of the pass. The end sections starting from the points of inflection f extend both toward the corresponding entering circumference c of the work and toward the corresponding centerline r of *the adjacent radial gap. Each end area lies within a rst 'center angle al and outside a second center angle a2, both the first and second center angles having as a common leg the centerline of the radial gap between two neighboring 'end sections of adjacent zones, which irst center angles 'are respectively 24, 22, 20, 18, 15 and 12, and 'the second center angles are respectively 7, 5, 4, 3, 'and 2 when the numbers of the zones are respectively 2, 3, 4, 5, 6, and 7.

There is thus defined between neighboring end sections of adjacent rolls, as viewed in the plane of the drawings which is the rolling plane, a wedge-like space, lthe sides ofthe space being constituted by the end sections. The smallest radial distance of the said end sections from 'the longitudinal axis of the pass is the same las the largest 'Aradial distance of the adjoining middle section from the axis, the largest radial distance of said adjoining middle 'section being the inflection point f. A

By entering circumference is meant the circumference 'of the work as it enters the pass, and by the centerline 'of the radial gap is meant the straight radial line which lies in the middle of the roll gap. Y The work is subjected to oblique rolling forces from 'both sides as it passes between neighboring end sections of the Vadjacent rolls. These forces performy the work 'of deformation along lines` of force, whereby these forces 'progressively decrease. The force acting at each point of the lines of force has components in the longitudinal ldirection of the work, in the Itransverse direction, and in 'the radial direction toward the mandrel. In FIG. -1 there 'are shown by way of example only four lines of force '1-1, 2 2, 3-3, tand 4-4, as well as the direction of the 'components of the forces starting at the points 1, 2, 3 and '4 of the neighboring end sections. VThe oblique forces intersecting .each other on the centerline r of the gap have a resultant directed in the radial direction toward the mandrel. In order not to make the ligure too complicated, only a single point is indicated within the work material, in the course of the line of force 2 2, by

Vshowing the radial and transverse direction of the components of force acting in the said point. The forces divert a considerable part of the spread as it develops in the longitudinal direction of the Work, leaving only such a small part of the spread in the transverse direction that the ribs produced remain very lsmall'even for the largest stretchings, and do not extend, even in passes composed of the conventional number of rolls, to the respective entering circumferences of the work. In this Way the danger of fin formation is completely eliminated despite the maximum longitudinal stretchings. The said rolling of the work between neighboring end sections adds to the large longitudinal stretchings of the work. Furthermore, the work opposite the roll gaps is pressed against the mandrel, so that the separations of the work from the mandrel, which are familiar in prior art rolling techniques,

do not occur. This is important, because eicient rolling can not take place between the rolls and the mandrel if the work is not pressed against the mandrel.

The prolile of the rolls of the passes according to the present invention is best if the profiles of the predominant middle sections are circular with respect to the longitudinal Aaxis of the work. However, very favorable results can still be achieved if the profiles of the middle sections of the rolls do not lie entirely on theinscribed circle, but are concave, dilfering from the inscribed circle slightly toward their ends in that the points of inflection from which the end sections extend lie somewhat outside the respective inscribed circle. The distance outside the inscribed circle should at the most be one-fourth the difference between the radius of the inscribed circle of the pass and the circle inscribed in the circumference of the work entering the respective pass.

The method according to the invention is carried out in a plurality of passes in which the rolls are circumferentially offset from the rolls in the preceding and succeeding passes, so that the ribs` formed in `one pass will be acted on by the middle sections of the rolls of a succeeding pass. The hollow body is stretched on the mandrel b, which extends through all of the passes, and the rolls of the passes can be either idle or driven rolls having the profiles as described above. According to the invention, the shortest distance of the middle sections of the rolls from the longitudinal axis of the passes is at most 0.83 time the shortest distance from the entering circumference of the work to that axis, plus 0.17 time the radius of the mandrel. Each pair of adjacent end sections f-g constitute, when viewed in the rolling plane, the sides of a wedge-like space, or of an acute angle wedge-like space, from the sides of which the oblique rolling forcesare exerted for stretching the parts of the work lying between adjacent middle sections of the rollsas well as the ribs developing on them in the longitudinal direction, and thereby reducing the growth of the developing tribs and preventing their reaching the entering circumference of the work.

In the passes for carrying out the method according to the invention, the very large individual stretchings can be maintained at an equal level, despite the continuous decrease in the wallthickness of the work, although the ratio of the wall thickness to the diameter of the mandrel is `continuously decreasing in ever larger steps when stretchings of equal size are performed in succeeding passes. Hence the method is outstanding because it reduces in considerably fewer passes than are necessary in present practice the large wall thickness of hollow bodies to the prenished wall thickness of the work, namely to a wall thickness (which is still larger than the finished wall thickness). produced in a pass precedingthe last pass or passes of the series, in which the finished wall thickness of the work is reached on the circumference of the work or on parts thereof. The spread developed in each of the passes according to the invention is extremely small, even for the largest stretchings, and therefore the ratio of the Volume of work material displaced in the longitudinal direction to the volume displacedA as spread in the lateral direction is much greater than in the passes used in practice up to now. The ratio of the said volumes is the greatest in passes composed of 5 to.7 rolls, in which the wlthdrawals of work material occurring at the roll gaps are greater than in passes composed of the usual number of rolls. Nevertheless, the spread is so small, even where the passes are composed of the usual number of rolls, that the work material also withdraws in such passes at the roll gaps, thus eliminating any danger of fin formation.

The applicant has already mentioned that in rough hot rolling mills, therpasses are never accurate, and furthermore the wear of the rolls in the different passes, as well as of the mandrel, is irregular. For these reasons, certain tolerances in the dimensions of the passes are necessary. The technical delivery conditions of seamless tube works provide in many countries, inter alia, a plus tolerance of 15% in the nominal wall thickness of hotnished seamless tubes. The characteristic set forth above for the minimum distance of the middle zonesof the rolls in the passes from the longitudinalaxis of the work, is already so computed that a tolerance of 5% is included therein. The said tolerance of 5% means that the ratio of the smallest wall thickness of the Work entering a pass to its smallest wall thickness leaving the pass, is at the most 5% smaller than the minimum geometric mean ratio nv .s/sn: 1.26

If the geometric mean ratio in a series of passes were exactly 1.26, then the above-mentioned ratio in one or the other of the passes could be up to smaller than 1.26, which slight difference would be compensated by the other passes in which the said ratio would be greater than 1.26, without being bound to any upper limit. The small tolerance of 5% included in the characteristic of the passes is not only necessary, but is also permissible, because despite the tolerance, the stretching performed in such a pass remains larger than performed in passes used in todays practice, and also because the spread in thev pass remains so small that withdrawals occur at the roll gaps, even in passes composed of the usual number of rolls, thus eliminating any danger of fin formation,

FIG. 1 of the drawing also shows, for purposes of comparison, two passes of the prior alt, one having rolls m which are concave only, indicated in broken lines, the radius of the profile of which is greater than the radius of the round entering work, the end of the concave profile lying on the entering circumference of the work, and the other pass having rolls n, shown in dot-dash lines, the profile of which is convex only, and the ends of the profile lying on the entering circumference of the work.

The middle sections of the rolls e according to the present invention penetrate deeply into the work and produce predominant parts of the circumference of the work with a uniform minimum wall thickness of less than 0.83 of the wall thickness of the entering wor while the end sections penetrate to the same depth as the middle sections of the rolls. Hence the parts of the work lying between neighboring end sections of adjacent rolls are also efficiently rolled within the entering circumference of the work. VThe latter can best be noted from the large amount of work material displaced by the end sections. ln terms of total work material displaced, the rolls e displace substantially more than the rolls m or the rolls n, and therefore produce a much larger stretching than the latter rolls. The obliquely directed rolling forces 1 4 acting in the region of the neighboring end zones of adjacent rolls e press the work material against the mandrel and thereby prevent the occurrence of local separations from the mandrel, which separations are known to occur in conventional methods. The end sections subject the parts of the work between adjacent middle sections with the ribs developing on them to stretching by rolling, so that a considerable part of the spread is diverted from the transverse direction to the longitudinal direction of the Work. As a consequence, the remaining part of the spread becomes too small that the small ribs have more than sufficient space, even for the largest stretchings produced, so that they lie within the entering circumference of the work and do not reach the same, whereby the danger of iin formation is completely avoided, even in passes composed of the usual number of rolls.

In contradistinction thereto, the stretchings in passes of rolls which have only concave profiles, such as those of the rolls m, cannot, because of the danger of fin formation, be increased beyond a certain limit. Since scarcely any rolling within the entering circumference of the work takes place toward the ends of the roll profiles, the most important prerequisite for eticiently reducing the developing spread is missing.

FIG. 2 shows a six roll pass with the work being rolled therein, the Work entering the pass having previously been rolled in a similar pass. In the drawing, a is the work rolled in the pass, and b is the mandrel. The circumference of the Work entering the pass is indicated by the .broken line c. The work enters the pass with a circular shape with respect to the longitudinal axis of the work along most of the circumference of the work. The circumference of the completely -nished work after it is rolled in all the passes is indicated by the dot-dash line d.

This shows that the smallest wall thickness produced in the pass shown in FIG. 2 is greater than the finished wall thickness to which the Work is to ibe reduced. The profile of the rolls e, as in FIG. l, consists of the middle section f-f which is circular with `respect to the longitudinal axis of the work. The end points f of the middle sections are points of inflection, and they lie within the entering circumference of the work. At these points end sections f-g, preferably of convex shape, are joined to the middle sections, and the end sections extend toward the entering circumference and toward the centerline 0-X of the roll gap. The ligure shows that between the adjacent end sections of the rolls, small ribs h develop within the entering circumference of the Work, which ribs can easily be rolled down in a succeeding offset pass. The said ribs are so small that they do not reach the entering circumference c of the work. Under the conditions shown in the drawing, the reduction in cross section of the work is sosubstantial that a stretching of about takes place within the pass.

The convex shape of the end sections is most advantageous, because with such a shape the ribs are given the smallest size. However, satisfactory results can be achieved if the end sections are given a slightly different shape. As shown in FIG. 3, the proles of the rolls e' have a convex middle section f-f, as with the rolls in FIGS. 1 and 2. 'The end sections 1"-g' however extend from the point of iniiection f' first curving outwardly toward the entering circumference of the work, and then extending straight to the end point g of the end section. Likewise, in FIG. 4, the proiiles of the rolls e" have a convex middle section f"-f", and the end sections f-g extend substantially straight from the point of inflection f" to the end of the roll protile g. The roll profile at the point of inection f may be slightly 4rounded oif.

With the. applied method,large stretchings can be obtained in a relativelyV few passes, whereby the work is rolled very uniformly on the major part of its circumference, and the spread is so small that the danger of iin formation is completely eliminated, even in passes composed of the usual number of rolls. These advantages represent a substantial technical and economical advance in the art over the methods used in practice up to now.

With the invention, substantial progress can be achieved in existing plants for stretching hollow bodies on a mandrel, if the previously used passes or a part of them are replaced by the passes of the present invention. The progress will still be noteworthy if one or another previously Iused pass is usedbefore, after, or within the plurality of pass according to the invention. It is also possible to divide the passes used to perform the present method among two or more appropriate machines.

I claim:

1. A method of stretching hollow metallic bodies, especially seamless tubes, comprising suporting the work against a decrease of its internal diameter, applying rolling pressure against the circumference of the work a plurality of n times and at intervals separated from each other in the direction of the length of the Work, for accomplishing elongation of the work by reducing its wall thickness to a pre-finished wall thickness, the first application of pressure being against the work when it has a smallest wall thickness so, the smallest wall thickness so of the work prior to the tirst application of rolling pressure being reduced in n steps to a smallest wall thickness amountsto at least 1.26 which expression is the geometric mean-ratio of n individual ratios, each step having an individual ratio of the smallest Wall thickness preceding the step to the smallest wall thickness after the step has been performed, and each application of rolling pressure being directed against a plurality of zones of the circumference of the work, said zones being spaced around the circumference of the Work and together constituting sub stantially the whole circumference of the work, each zone after each application consisting of a predominant middle section having a profile substantially circular with respect to the longitudinal axis of the work, and said zones each further consisting of two end sections Yjoined with the ends of the middle section in points of inflection, each pair of neighboring end sections of adjacent zones constituting the flanks of small ribs having a prole ranging from a prole which is concave with respect to the interior of the ribs to a prole which is a straight pro-V le, the proles of the end sections extending from the points of inflection towards the respective circumference of' the work against which rolling pressure is to be applied, as well as toward the nearest axis of symmetry b.,- tween pairs of adjacent end sections, and rolling pressure being exerted at the flanks of the developing ribs from both sides in oblique directions against the parts of the work lying between adjacent middle sections, for turning a considerable part of the spread as it develops from ,the lateral direction to the longitudinal direction of the work, and for thereby decreasing the part of the spread remaining in the lateral direction to restrictthe development of the said ribs. so that they are harmless, the ribs being small enough so that they do not, even at the largest stretchings, reach the circumference of the work prior to the respective application of rolling pressure.

2. An apparatus for stretching hollow metallic bodies, especially seamless tubes, comprising a mandrel and a plurality of angularl'y staggered roll passes, through which the work carried on the mandrel is driven, the rolls in each pass 4being separated from each other by narrow gaps, and the profile of each roll in each pass consisting of a predominant middle section having a profile substantially circular with respect to the longitudinal axis of the t pass, the smallest radial distance of the middle section from the longitudinal axis of the pass being at the most 0.83 times the smallest radial distance of the circumference of the work entering the pass from the said axis plus 0.17 times the radius of the mandrel, and said roll profile further consisting of two end sections joined to the ends of the middle section in points of inflection, and each end section having a proiile convex with respect to the body of the respective end part of the roll, the prolile of each end section extending from the point of in ection ever farther from the longitudinal axis of the pass as Well as toward the radial middle line of the nearest roll gap, each pair of neighboring end sections of adjacent rolls constituting, as seen in the rolling plane, the sides of a wedgelike space.

3. An apparatus for stretching hollow metallic bodies, especially seamless tubes, comprising a mandrel and a plurality of angularly staggered roll passes, through which the work carried on the mandrel is driven, `the rolls in each pass being separated from each other byV narrow gaps, and the proiile of each roll in each pass consisting of a predominant middle section having a profile substantially circular with respect to the longitudinal axis of the pass, the smallest radial distance of the middle section from the longitudinal axis of the pass beingat-the most 0,83 times the smallest radial distance of the circumference of the work entering the pass from the said axis plus 0.l7,times the radius of the mandrel, and said roll profile further consisting of two end sections joined to the ends of the middle section in points of inection, and each end section having a profile partly convex with respect to the body of the respective end part of the roll, the profile changing 4to straight, the protile of each end section extending from the point of iniiection ever farther from the longitudinal axis of the pass as Well as toward the radial middle line of the nearest roll gap, each pair of neighboring end sections of adjacent rolls constituting, as seen in the rolling plane, the sides of a wedge-like space.

4. An apparatus for stretching hollow metallic bodies, especially seamless tubes, comprising a mandrel and a plurality of angularly staggered roll passes, through which the work carried on themandrel is driven, the rolls in each pass being separated from each other by narrow gaps, and the profile of each roll in each pass consisting of a predominant middle section having a profile substantially circular with respect to the longitudinal axis of the pass, the smallest radial distance of the middle sec- .tionfrom the longitudinal axis of the pass being at the most 0.83 times the smallest radial distance of the circumference of the work entering the pass from the said axis plus 0.17 times the radius of the mandrel, and said roll prolefurther consisting of two end sections joined to the ends of the middle section in points of inflection, and each end section having a straight profile, the profile of each end section extending from the point of inilection ever farther `from the longitudinal axis of the pass as Well as toward the radial middle line of the nearest roll gap, each pair of ,neighboring end sections of adjacent rolls constituting, as seen in the rolling plane, the sides of a wedge-like space.

References Cited by the Examiner UNITED STATES PATENTS 721,209 A 2/03 Mannesmann 80--62 1,858,990 5/32 Foren 80-62 FOREIGN PATENTS 215,932 6/61 Aust-ria. 203,009 8/23` Great Britain. 789,155 1/58 Great Britain.

WILLIAM- I. STEPHENSON, Primary Examiner. LEON PEAR, Examiner.

UNITED STATES-PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3,176,493 April 6, 1965 Eugene Weiss It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as correctedbelow.

Column l, line 30, beginning with "the present in-", strike out all to and including "is a middle section" in line 6l, same column l, and insert the same before "which" in column Z, line l; Column 6, line 65, for "suporting" read supporting Signed and sealed this 28th day of September 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A METHOD OF STRETCHING HOLLOW METALLIC BODIES, ESPECIALLY SEAMLESS TUBES, COMPRISING SUPPORTING THE WORK AGAINST A DECREASE OF ITS INTERNAL DIAMETER, APPLYING ROLLING PRESSURE AGAINST THE CIRCUMFERENCE OF THE WORK A PLURALITY OF N TIMES AND AT INTERVALS SEPARATED FROM EACH OTHER IN THE DIRECTION OF THE LENGTH OF THE WORK, FOR ACCOMPLISHING ELONGATION OF THE WORK BY REDUCING ITS WALL THICKNESS TO A PRE-FINISHED WALL THICKNESS, THE FIRST APPLICATION OF PRESSURE BEING AGAINST THE WORK WHEN IT HAS A SMALLEST WALL THICKNESS SO, THE SMALLEST WALL THICKNESS SO OF THE WORK PRIOR TO THE FIRST APPLICATION OF ROLLING PRESSURE BEING REDUCED IN N STEPS OF TO THE SMALLEST WALL THICKNESS SN AFTER THE LAST APPLICATION OF ROLLING PRESSURE, THE RATIO SO/SN BEING SUFFICIENTLY LARGE THAT THE EXPRESSION
 2. AN APPARATUS FOR STRETCHING HOLLOW METALLIC BODIES, ESPECIALLY SEAMLESS TUBES, COMPRISING A MANDREL AND A PLURALITY OF ANGULARLY STAGGERED ROLL PASSED, THROUGH WHICH THE WORK CARRIED ON THE MANDREL IS DRIVEN, THE ROLLS IN EACH PASS BEING SEPARATED FROM EACH OTHER BY NARROW GAPS, AND THE PROFILE OF EACH ROLL IN EACH PASS CONSISTING OF A PREDOMINANT MIDDLE SECTION HAVING A PROFILE SUBSTANTIALLY CIRCULAR WITH RESPECT TO THE LONGITUDINAL AXIS OF THE PASS, THE SMALLEST RADIAL DISTANCE OF THE MIDDLE SECTION OF THE LONGITUDINAL AXIS OF THE PASS BEING AT THE MOST 0.83 TIMES THE SMALLEST RADIAL DISTANCE OF THE CIRCUMFERENCE OF THE WORK ENTERING THE PASS FROM THE SAID AXIS PLUS 0.17 TIMES THE RADIUS OF THE MANDREL, AND SAID ROLL PROFILE FURTHER CONSISTING OF TWO END SECTIONS JOINED TO THE ENDS OF THE MIDDLE SECTION IN POINTS OF INFLECTION, AND EACH END SECTION HAVING A PROFILE CONVEX WITH RESPECT TO THE BODY OF THE RESPECTIVE END PART OF THE ROLL, THE PROFILE OF EACH END SECTION EXTENDING FROM THE POINT OF INFLECTION EVER FARTHER FROM THE LONGITUDINAL AXIS OF THE PASS AS WELL AS TOWARD THE RADIAL MIDDLE LINE OF THE NEAREST ROLL GAP, EACH PAIR OF NEIGHBORING END SECTIONS OF ADJACENT ROLLS CONSTITUTING, AS SEEN IN THE ROLLING PLANE, THE SIDES OF THE WEDGELIKE SPACE. 